1. Field of the Invention
This invention relates to a multiplex transmission system for carrying out a data transmission among a plurality of multiplex nodes connected to a common multiplex transmission line, and more particularly, to a multiplex transmission system using CSMA/CD (carrier sense multiple access/collision detection).
2. Description of the Related Art
In conventional multiplex transmission systems of this type, a plurality of terminals (multiplex nodes) are connected to a common multiplex transmission line (multiplex bus) formed of, e.g., a paired cable. One of the multiplex nodes sends a predetermined data frame to the multiplex bus in accordance with the CSMA/CD method, to thereby transmit data simultaneously to the other multiplex nodes. The data frame includes an acknowledge signal (ACK signal) area at an end thereof.
In such systems, it is known that each of the multiplex nodes that received the data frame returns an ACK signal to a bit position of the ACK signal area preassigned thereto.
The multiplex nodes each include a computer for communication, and these computers are operated independently of one another and are able to send a data frame to the multiplex bus at a desired timing. Therefore, a collision of transmitted messages can occur on the multiplex bus, and conventionally this is prevented by setting a priority sequence for the transmission of individual messages.
In accordance with the priority sequence thus set, each computer sends a message therefrom while carrying out a priority control so that the message is not destroyed. Specifically, each computer carries out a data transmission according to a nondestructive arbitration type CSMA/CD access method in which the transmission of a message with a lower priority is automatically interrupted and only the higher-priority message is continuously transmitted. When the data is properly received, each multiplex node returns an ACK signal to a location of an address uniquely assigned thereto (respective bit area in the ACK signal area).
If a local error occurs in the network and the data received by a multiplex node is erroneous, then no ACK signal is returned from this multiplex node, and therefore, the sending multiplex node determines that an abnormal situation occurred, and retransmits the data. The sending multiplex node repeats the transmission of the data, e.g., three times at the maximum, until all of the registered multiplex nodes return their ACK signals.
A sending multiplex node uses an ACK management function, i.e., if a node fails to return the ACK signal even though the data transmission was repeated three times, such a node is regarded as being in trouble and is excluded from the registered nodes, and when an extra ACK signal is received, the node corresponding to this ACK signal is registered.
In the above-described multiplex transmission system, data transmitted from each multiplex node constitutes an independent frame, and therefore, where data from a plurality of multiplex nodes must be simultaneously transmitted to a certain multiplex node, the arrival times of individual frames vary significantly due to a variation in the delay times resulting from the traffic dependency. This phenomenon can cause a drawback in the case wherein an arithmetic operation must be carried out based on the received data to perform a different control on various devices. Namely, the intended control cannot be executed.
To eliminate this drawback, a time slot method in which data from a plurality of multiplex nodes is collected into one frame may be used. In this method, a synchronizing signal, such as a synchronizing pulse indicating a data boundary, must be generated by a specific node, and thus a centralized control system must be constructed. Therefore, this method cannot be applied to the case in which data obtained simultaneously from a plurality of multiplex nodes is used by a plurality of multiplex nodes to carry out a multifunction control. That is, a send request for collecting simultaneous data as required cannot be produced by an arbitrary multiplex node.
A CSMA/CD transmission system based on a bit-by-bit contention technique may be constructed in such a manner that a subnode sends data to a portion of the frame generated by a main node. Nevertheless, such a system has a problem of how to synchronize a plurality of multiplex nodes and how to carry out an error checking on the gathered data.